In the liquid chromatographic separation of mixtures into two or more components on a commercial scale, it is economically desirable to operate under conditions that maintain low elution volume to feed ratios. The reason for this is mainly due to the cost involved in distilling the carrier and eluent from the feed components after they have been eluted from the chromatographic column; this cost increasing as the elution volume to feed ratio increases. One of the main causes of high elution volumes in these processes is related to the phenomena that peaks which are eluted last from the column have larger half widths than those eluted early.
Chromatography is a process whereby components contained in a fluid mixture may be separated from the mixture. This is accomplished by the selective retardation of one or more of the components of the fluid mixture as the fluid uniformly moves through a column containing a stationary substrate. The retarding results from the distribution of the components of the mixture between the substrate and the bulk fluid as the fluid moves past the stationary phase.
One of the particular chromatographic methods employed is called elution chromatography. In this technique, the feed mixture and carrier fluid plus eluent are passed sequentially through the column. In this specification, carrier fluid is taken as liquid materials which are not significantly adsorbed by the substrate in the presence of feed mixture components. Eluent is a term to describe liquid materials which are adsorbed by the substrate and compete for adsorption sites with the feed components. This sequential passage of carrier-eluent and feed leads to a differential migration of the feed's components according to their distribution between two phases. If the components of the sample have different distribution coefficients, a separation of the components is achieved as the components will elute in sequence from the end of the stationary phase. In ordinary elution development, there is a small range of retention volumes or retardation factors for optimum separation. The distribution coefficients must be sufficiently large so that the components eluted early are not pushed off the column as an unresolved series of bands, yet the distribution coefficients must be reasonably small if excessive elution times and peak broadening are to be avoided. In addition, there must be a difference in the distribution coefficients of the component in order to effect their separation.
In prior copending application Ser. No. 386,949, a technique for the automatic gradual attainment of the elution power required for each component's separation was described as well as the benefit of operating this technique at higher temperatures. It has now been discovered that by preheating the stronger eluent-carrier mixture before its passage through the bed that even smaller elution volumes are obtained. This advantage of course means lower total elution volume to feed ratios, thus providing an improved gradient elution process which produces paraxylene at substantial savings.